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Installing transom-mounted transducers can be tricky

By John Barry

Transducers come in lots of shapes and sizes.  Generally, there are three types of transducer configurations, thru-hull, in-hull and transom mounted.  One of the standards the NMEA produces—called the NMEA 0400 Installation Standard--includes a compatibility matrix that shows which transducers may be used with which hull types.  For instance, a bronze transducer is incompatible with an aluminum hull.  For trailered boats, the transom-mounted transducer is usually the best choice. Here are some basics to keep in mind.

The state that has the most boat registrations is not Florida—it’s Michigan!  This is because small trailer boats are abundant and Michigan has 3288 miles of coastline compared to 1350 miles for Florida.  The preponderance of small boats means a preponderance of small-boat marine electronics and this includes transom-mounted transducers.  Trailered boats generally use transom-mounted transducers to allow the hull to sit on trailers’ bunkers without risking the transducer.  Installing a transom-mounted transducer is tricky.

First of all, the transom is a very turbulent place and transducers hate turbulence.  This means that transducer placement is critical.  For single-screw boats we install the transducer on the starboard side to minimize turbulence from the prop.  For twin-screw vessels, we place the transducer close to the centerline and as deep as possible.  Placing the transducer at the correct spot on the transom is a good start, but there is more to a transom-mounted installation.

When a boat goes through the water, the action of the water against the hull is quite complex.  Cavitation bubbles form along the hull, behind strakes, around strainers, and even from the bumps in the paint.  Depending on speed, turbulence below the boat can be severe.  Stepped hulls on go-fast boats use cavitation to allow the boat to lose friction with the water and gain speed.  All hulls get some turbulence as they displace the water and propel forward.

Understand the boat’s motion
Installing a transom-mounted transducer requires that the installer understand the motion of the vessel.  Manufacturers’ instructions tell us to tuck the top of the transducer above the transom while the bottom is well below the transom.  The instructions also tell us to aim it at the seabed bottom.  Lastly, we are told to keep the "trailing edge deeper than the leading edge.”  This translates to slightly bow up.  All of this geometry is about allowing the water to flow smoothly across the transducer’s face without cavitation bubbles or turbulence.  A properly installed transom-mounted transducer will perform extremely well.  Boats that go very fast—over 20 knots—may have issues with transom mounts.

Transom-mounted transducers mount with a bracket to the transom.  The bracket allows us to adjust the face of the transducer to aim straight at the seabed.  This bracket has a kick-out feature to protect the transducer if it is bumped by the boat trailer.  Clearance must be maintained to allow the kick-out bracket to function. This bracket has a release force and can kick out at speeds over 40 knots. Really fast boats usually use an in-hull transducer.

Transducers vary in physical size and shape as well as in electrical characteristics.  The traditional 50 and 200 kHz frequencies are still in use and there are many other frequencies being used these days.  Basically, lower frequencies are more powerful for penetrating deep water, so we use low frequencies to mark deep water.  Higher frequencies produce better resolution, so we use higher frequencies to mark bottom details and fish.  Transducers also have a beam width associated with them. Narrow-beam transducers are fine for digital depth and deep water, over 1000 feet. Wider beam transducers are needed for shallow water since a wider beam marks a wider path.

Transducers are changing quickly.  With the advent of CHIRP, multi-beam and multi-frequency transducers, the shape of transom-mounted transducers has changed.  Always follow manufacturers instructions over hints given here.  Selecting the correct transducer for the vessel is the most important thing.  Transom mounts fit more boats than any other configuration.  Getting the geometry right is essential.  A properly installed transducer finds fish and skinny water better—and increases safety and enjoyment.

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